In cellular communications systems, a multiplicity of base stations may be distributed throughout a geographical area. Each base station communicates with user equipment devices, such as mobile communications devices, which may move throughout an area served by the base station.
In order for a device to properly decipher signals that it receives from a base station with which it is currently communicating, the user equipment needs to know how received signals may be affected by environmental conditions and other multipath considerations. The mechanism for determining these affects is known as “channel estimation”. In order to provide channel information, a base station may periodically transmit, at known times and/or at known frequencies, “pilot” signals which are defined by a known pattern of bits. Pilot signals may also be referred to as “reference signals” (RS), and these terms are used interchangeably throughout the following disclosure.
A pilot signal pattern may be based, for example, on a particular pseudo-random-noise (“PN”) sequence. By comparing a received pilot pattern to an expected pilot pattern, channel qualities may be determined. The pilot pattern, in the general case, may also be based upon some scrambling/spreading sequences. Different base stations may utilize different PN sequences to facilitate identification of the base station from which a pilot signal is received.
In compliance with emerging LTE (Long Term Evolution) standards being promulgated by the 3GPP (3rd Generation Partnership Project) the pilot signals associated with each base station are transmitted in the same OFDMA (orthogonal frequency division multiple access) time/frequency bin. Thus, there may be strong interference from the reference signals transmitted by neighboring base stations (or by the multiple transmitters of a base station, if a base station has more than one transmitter).